They get up onstage or they appear onscreen and they switch it on, an ineffable quality that makes everyone love them.

Michael J. Fox is one of those guys. Always has been, evidently always will be.

Not even his very public battle with Parkinson’s disease can rob him of it.

More than 25 years after being diagnosed with the degenerative disorder of the central nervous system, Fox still possesses the same magic as the star of The Michael J. Fox Show, a family comedy that’s one of the best new shows of the 2013-14 season.

It premieres at 8 p.m. Thursday on NBC with back-to-back episodes.

Fox never disappeared from the airwaves, thanks to a series of recurring and guest roles in such shows as Scrubs, Boston Legal, Rescue Me, Curb Your Enthusiasm and The Good Wife.

But this is the 52-year-old actor’s first full-time TV gig in more than 10 years, since he walked away from the leading role in Spin City in 2000.

“When I was diagnosed with this in 1991,” he recently told Emmy magazine, “they told me I had 10 years left to work. The guest shots were a proving ground. I was putting my toe in the water, thinking, ‘When do I drown? And when does that shark come?’ And the shark never came.”

Finally, after years of wading hip-deep in those waters, Fox decided it was time go all the way in again.

“I have the chance to do a show,” he reasoned. “Why am I not doing what I do?”

There’s a story about the actor that the late Gary David Goldberg, creator of Family Ties, the 1982-89 sitcom that made Fox a star, liked to tell:

“We would watch the studio audience and note their reaction to him,” Goldberg said. “When he would be onstage, even in the earliest episode of Family Ties [in which he played eldest son Alex P. Keaton], you could literally see the audience collectively lean forward in their seats. Then, when he would exit, they would lean back, just a little bit, maybe without even realizing they were doing it.

“Clearly, right from the very beginning, he was really intriguing to them.

“Years later, when we did the research for the initial pilot for Spin City [the 1996 comedy in which Fox played the deputy mayor of New York City], viewer reaction to Michael’s character was all over the map. Half of the people in the focus groups felt he had the best interest of the city at heart, was altruistic, cared about the mayor, cared about everyone — and they really, really liked him. The other half felt he was self-serving, conniving, only out for himself — but amazingly, they still liked him!

“It literally didn’t matter whether they saw him as a good guy or a bad guy. They just liked him.”

All these years later, Fox is still blessed with that can’t-buy-it/can’t-learn-it/can’t-fake-it appeal.

And The Michael J. Fox Show is an ingenious way of bringing him back.

The premise addresses his Parkinson’s without the story being overtly autobiographical. What happens onscreen is more like an upbeat, slightly distorted, funhouse-mirror version of Fox’s life.

He plays Mike Henry, a beloved New York news anchor with Parkinson’s who called it quits five years ago to focus on his health and to spend more time with his wife and three kids.

Maybe it would be easier for him to lead a somewhat normal life if an adoring city didn’t applaud him and high-five him for his heroic fight. “Oh, those sons of b------!” wife Annie (Betsy Brandt) says, gently mocking Mike when he complains about the daily standing ovations.

When Mike decides to go back to work, the station manager (Wendell Pierce) pulls out all the stops with a manipulatively sentimental advertising campaign, one that’s not unlike NBC’s ad campaign trumpeting Fox’s heroic return to television.

The funniest moment in the pilot comes when Mike accidentally dials 911 on his phone instead of the 917 area code, then exacerbates an awkward situation by explaining to the operator that “My drugs haven’t kicked in yet.” Suffice it to say that the police soon arrive at his doorstep.

But after the opening half-hour, in which Parkinson’s, the elephant in the room, is dealt with in a self-deprecating way, the two subsequent episodes seem more inclined to make a statement by making no statement at all.

Parkinson’s, the show suggests, can be just a part of life — one doesn’t have to be defined by it.

Many viewers, those who have been rooting for Fox for years, especially after he became a national champion for Parkinson’s awareness, will likely see this as a courageous career move.

“People said, ‘Are you sure you can handle this?’” he told Rolling Stone. “I said, ‘No, I’m not sure, but I want to and I have an opportunity to. And another side of it, it might be empowering for people.”

Granted, there are always going to be people who, in this era of political correctness, don’t approve of laughing in the face of serious illness.

But as Fox told Emmy, “What’s the other option? Crying about it, which doesn’t make sense to me. If you laugh at something, you take the sting out of it. You take the power out of it.

“I’m not making fun of Parkinson’s or even myself. I’m just saying, these are the realities of my world and this is how I choose to look at it: through humor.”

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